The present invention relates to a method for producing hot-dip galvanized high-strength steel sheets (including hot-dip galvannealed high-strength steel sheets) which are suitable for use as automotive inner panels, outer panels, etc.
Recently, in view of safety, weight reduction, and improved gas mileage in automobiles, and also in view of improvement in the global environment, there is a growing tendency to use hot-dip galvanized high-strength steel sheets as automotive steel sheets.
In order to produce a hot-dip galvanized high-strength steel sheet, the steel sheet must have superior galvanizability and must have the desired strength and workability after the steel sheet passes through a molten zinc bath, or after the steel sheet is further subjected to galvannealing.
In general, in order to increase the strength of a steel sheet, solid solution hardening elements, such as Mn, Si, and P, and precipitation hardening elements, such as Ti, Nb, and V, are added thereto. It is known that when a steel sheet to which such elements have been added is treated in a continuous galvanizing line (CGL), galvanizability is deteriorated.
Since the amounts of the alloying elements inversely affect the strength and the galvanizability, it has been extremely difficult to produce a hot-dip galvanized high-strength steel sheet having superior galvanizability in the continuous galvanizing line. Additionally, since the hot-dip galvanized high-strength steel sheet generally has inferior characteristics regarding workability, such as in elongation, it has been more difficult to produce a hot-dip galvanized steel sheet having superior workability.
As a conventional high-strength steel sheet having improved workability, a steel sheet with a complex structure, in which a ferrite matrix contains a low-temperature transformed phase having martensite as a principal phase (also including retained austenite), is known. The steel sheet with this complex structure has non-aging properties at room temperature and a low yield ratio, and has superior workability and superior bake hardenability after working. The steel sheet with a complex structure is produced by heating at temperatures in the ferrite and austenite (xcex1+xcex3) two-phase region, followed by quenching by water-cooling, gas-cooling, or the like.
However, when the steel sheet with a complex structure is galvanized at a temperature of approximately 500xc2x0 C., or is further galvannealed, martensite distributed in the ferrite matrix is tempered, tensile strength and elongation are decreased, and the upper yielding point appears, resulting in an increase in yield ratio, and also yield point elongation occurs.
Temper softening easily occurs as the amounts of alloying elements, such as Mn and Si, are decreased. On the other hand, when the amounts of such alloying elements are increased, hot-dip galvanizability is decreased. Ultimately, in the steel sheet with a complex structure, since martensite is tempered in the galvanizing process, it has been difficult to make workability and high strength, which are characteristics thereof, compatible with each other and also to develop satisfactory galvanizability, using the conventional techniques.
Accordingly, the applicant of the present invention has applied for other patents under International Application Nos. PCT/JP99/04385 and PCT/JP00/02547 for inventions relating to high-strength steel sheets having satisfactory galvanizability and methods for producing the same.
PCT/JP99/04385 is an invention relating to a high-strength steel sheet to which Mo and Cr have been added, which are significantly important in producing a dual-phase galvanized steel sheet with a complex structure in which the matrix ferrite contains the low-temperature transformed phase having martensite as the principal phase. However, Mo and Cr are very expensive elements and are constituents which are too costly for the production of general-purpose, inexpensive galvanized steel sheet to which the present invention is directed. Additionally, in PCT/JP99/04385, although Mo is added to the material containing a large amount of Mn in order to produce a more favorably dual-phase sheet steel with a complex structure, if Mo is added, the thickness of a band-like structure in the steel sheet is increased. Consequently, press cracking may occur, resulting in deterioration in workability, and in order to eliminate the band-like structure, high-temperature annealing is absolutely necessary. Although the high-temperature heating is effective for galvanizability when double heating is performed, the high-temperature heating acts adversely when single heating is performed, and thus it is not necessarily a condition suitable for to reconciling the two processes.
On the other hand, PCT/JP00/02547 relates to a galvanized steel sheet with a complex structure to which 1.0% to 3.0% of Mn and 0.3% to 1.8% of Si are added, and which contains the retained austenite phase and the tempered martensite phase which are very important in improving the strength-elongation balance. However, in order to obtain such a structure, a primary heating-cooling process and a secondary heating-cooling process must be combined. Additionally, in the cooling step after heating is performed in the primary process, quenching treatment must be performed rapidly at a cooling rate of 10xc2x0 C./s or more, down to the Ms temperature or less, resulting in processing difficulties. Also, in addition to a single heating-cooling process which is normally performed, at least one other heating-cooling process must be performed before the CGL line.
Accordingly, in order to overcome the problems associated with the conventional techniques described above, it is an object of the present invention to provide a method for producing a hot-dip galvanized high-strength steel sheet in which both satisfactory workability and high strength are provided, and moreover satisfactory galvanizability is obtained even if galvanizing is performed using facilities such as a continuous galvanizing line.
Specifically, it is an object of the present invention to obtain satisfactory galvanization while satisfying a TS of 590 MPa or more, an El of 25% or more, and a value of TSxc3x97El of 15,000 MPa.% or more, as standards for workability and high strength.
In this case, the present inventors have made every effort to carry out research to solve the problems described above and have discovered a hot-dip galvanized high-strength steel sheet having superior workability and galvanizability even if Mo and Cr are not added, and even if the retained austenite phase and the tempered martensite phase are not contained, as well as a method for producing the same, thus achieving the present invention.
(1) A hot-dip galvanized high-strength steel sheet having superior workability and galvanizability contains, in % by weight, 0.01% to 0.20% of C, 1.0% or less of Si, more than 1.5% to 3.0% of Mn, 0.10% or less of P, 0.05% or less of S, 0.10% or less of Al, and 0.010% or less of N, and also contains 0.010% to 1.0% in total of at least one element selected from the group consisting of Ti, Nb, and V, and the balance being Fe and incidental impurities, and also has the metal structure in which the areal rate of the ferrite phase is 50% or more, the ferrite phase has an average grain diameter of 10 xcexcm or less, and the thickness of a band-like structure composed of the second phase satisfies the relationship Tb/Txe2x89xa60.005, where Tb is the average thickness in the sheet thickness direction of the band-like structure and T is the thickness of the steel sheet.
(2) A hot-dip galvanized high-strength steel sheet having superior workability and galvanizability contains, in % by weight, 0.01% to 0.20% of C, 1.0% or less of Si, more than 1.5% to 3.0% of Mn, 0.10% or less of P, 0.05% or less of S, 0.10% or less of Al, and 0.010% or less of N, and also contains 0.010% to 1.0% in total of at least one element selected from the group consisting of Ti, Nb, and V, and further contains 3.0% or less in total of at least one of Cu and Ni, and the balance being Fe and incidental impurities, and also has the metal structure in which the areal rate of the ferrite phase is 50% or more, the ferrite phase has an average grain diameter of 10 xcexcm or less, and the thickness of a band-like structure composed of the second phase satisfies the relationship Tb/Txe2x89xa60.005, where Tb is the average thickness in the sheet thickness direction of the band-like structure and T is the thickness of the steel sheet.
(3) A method for producing a hot-dip galvanized high-strength steel sheet having superior workability and galvanizability includes the steps of hot-rolling a slab having the steel composition described in (1) or (2) above, followed by coiling at 750 to 450xc2x0 C.; optionally, further performing cold-rolling; heating the resulting hot-rolled sheet or cold-rolled sheet to a temperature of 750xc2x0 C. or more; and subjecting the hot-rolled sheet or cold-rolled sheet to hot-dip galvanizing while cooling from this temperature.
(4) A method for producing a hot-dip galvanized high-strength steel sheet having superior workability and galvanizability includes the steps of hot-rolling a slab having the steel composition described in (1) or (2) above, followed by coiling at 750 to 450xc2x0 C.; optionally, further performing cold-rolling; heating the resulting hot-rolled sheet or cold-rolled sheet to a temperature of 750xc2x0 C. or more; subjecting the hot-rolled sheet or cold-rolled sheet to hot-dip galvanizing while cooling from this temperature; and then performing galvannealing.
(5) A method for producing a hot-dip galvanized high-strength steel sheet having superior workability and galvanizability includes the steps of hot-rolling a slab having the steel composition described in (1) or (2) above, followed by coiling at 750 to 450xc2x0 C.; optionally, further performing cold-rolling; heating the resulting hot-rolled sheet or cold-rolled sheet to 750xc2x0 C. or more, followed by cooling; further heating to a temperature of 700xc2x0 C. or more; and subjecting the hot-rolled sheet or cold-rolled sheet to hot-dip galvanizing while cooling from this temperature.
(6) A method for producing a hot-dip galvanized high-strength steel sheet having superior workability and galvanizability includes the steps of hot-rolling a slab having the steel composition described in (1) or (2) above, followed by coiling at 750 to 450xc2x0 C.; optionally, further performing cold-rolling; heating the resulting hot-rolled sheet or cold-rolled sheet to 750xc2x0 C. or more, followed by cooling; further heating to a temperature of 700xc2x0 C. or more; subjecting the hot-rolled sheet or cold-rolled sheet to hot-dip galvanizing while cooling from this temperature; and then performing galvannealing.
That is, this may be accomplished by:
(1) positively adding at least one element selected from the group consisting of Ti, Nb, and V, it is possible to refine ferrite (xcex1) grains to 10 xcexcm or less due to pinning of the grain boundary migration of carbides, such as TiC, NbC, and VC, and also it is possible to suppress the coarsening of xcex3 gains generated and grown in the ferrite and austenite (xcex1+xcex3) two-phase region during heating or xcex3 gains in the austenite (xcex3) single-phase region;
(2) heating, the band-like structure composed of the second phase containing large amounts of C and Mn is dissolved so that the thickness of the band-like structure satisfies the relationship Tb/Txe2x89xa60.005, where Tb is the average thickness in the sheet thickness direction of the band-like structure and T is the thickness of the steel sheet.
Because of synergy between (1) and (2) described above, even without adding Mo and Cr, and also even if the structure does not contain the retained austenite phase and the tempered martensite phase, since the xcex3 grains before cooling are refined, the concentration of C and Mn from the xcex1 phase to the xcex3 phase during cooling is increased, the xcex3 phase is effectively transformed into martensite, and thus a hot-dip galvanized high-strength steel sheet having superior workability and galvanizability can be produced.
In particular, in contrast to PCT/JP99/04385 and PCT/JP00/02547, since Cr and Si, which are disadvantageous to galvanizability, are not substantially contained as essential elements, satisfactory galvanizability is obtained, and since Mo is not added, the band-like structure which is present before heating is relatively thin, and thus, even if high-temperature heating, which is disadvantageous in view of galvanizability, is not performed in the single CGL process, it is possible to produce a hot-dip galvanized high-strength steel sheet having superior workability.